the ability of qCuANOR to accept electrons from both menaquinol and cytochrome c551 may be related to the regulation of the rate of NO reduction especially as a defense mechanism of B. azotoformans against the toxicity of NO. The menaquinol pathway, which has a 4-fold greater maximal activity than the pathway via cytochrome c551, is used for NO detoxification, whereas electron donation via the endogenous cytochrome c involves the cytochrome b6f complex serving the bioenergetic needs of the organism